What format is your source media? It would help if you open one of your typical source files with MediaInfo, switch it to text view, copy the information and paste it here.
What is your delivery medium (e.g web video, DVD, Blu-ray etc.)? If web video then what resolution? If DVD or Blu-ray then for display in what territory?
Make sure you match your project properties to your source media properties. If that didn't happen automatically when you put your media on your timeline then use the "Match Media Video Settings" button at the very top right of your Project Properties window.